1. Field of the Invention
This invention relates to microencapsulating processes and the microcapsules produced therefrom.
2. The Prior Art
A method for the production of microcapsules containing oils using coacervation is disclosed in U.S. Pat. No. 2,800,457 (1957) to Green et al. The process described therein involves the coating of oil droplets with a liquid wall of gelatin-gum arabic colloidal material produced by coacervation. The liquid wall thus formed is hardened by treatment with formaldehyde. It is known that aqueous solutions of hydrophilic colloids such as gelatins or gum arabics can be caused to undergo coacervation by adding to said solutions various substances such as inorganic salts or oppositely charged colloids. The coacervate droplets are known to encase droplets of water-immiscible or slightly water-immiscible liquids and to gel and solidify upon cooling to room temperatures thereby forming liquid droplets encased in a gelatin membrane. After hardening of the gelatin membranes in the normal fashion (such as with formaldehyde) encapsulated suspensions of liquids are obtained which pg,3 after removal of water could be transformed by known drying methods into free-flowing dry powders. Other known patents teaching coacervation-related processes include:
U.s. pat. No. 3,244,640 (1966) to Studt et al. PA1 U.s. pat. No. 3,539,465 (1970) to Hiestand PA1 U.s. pat. No. 3,567,650 (1971) to Balcan PA1 U.s. pat. No. 3,594,326 (1971) to Himmel PA1 U.s. pat. No. 3,141,792 (1964) to Lachman et al. PA1 U.s. pat. No. 3,449,228 (1969) to Yurcheshen et al. PA1 U.s. pat. No. 3,779,942 (1973) to Bolles PA1 U.s. pat. No. 3,875,074 (1975) to Vassiliades et al.
Other patents teaching processes of producing oil containing microcapsules include: U.S. Pat. No. 3,242,051 (1966) to Hiestand teaching phase separation, U.S. Pat. No. 3,432,327 (1969) to Masao Kan et al teaching interfacial reaction between components of a dispersed phase and components of a continuous phase, U.S. Pat. No. 3,779,941 (1973) to Powell teaching the interfacial reaction of a water soluble and oil soluble resin, and U.S. Pat. No. 3,016,308 (1962) to Macauley teaching a method of producing microcapsules by spray drying an emulsion which contains a film-forming substance in the continuous phase.
All of the above described processes of microencapsulation suffer from one or more of the following disadvantages, particularly if commercial production of the resulting microcapsules, for use in aqueous coating compositions, is desired. Either the process requires a series of complicated steps including special apparatus and in most instances coacervation, or the microcapsules produced by the process cannot be conveniently used in aqueous coating compositions. In some of the above patented processes, most notably those involving interfacial polymerization, the wall thickness is limited because of the need for diffusion of at least one of the reactants through the microcapsule wall. The use of coacervation in a microencapsulation process suffers from other disadvantages in that the microcapsule product must be separated from the liquid phase and also that coacervation results in the production of clusters and aggregates of microcapsules as opposed to the more desirable production of discrete individual microcapsules.
The process of this invention includes a number of advantages not found in prior art methods. The materials used are relatively inexpensive and are readily available in commercial quantities. The process requires no complicated steps, simply emulsification of the desired droplet solution in the solution containing the wall forming compound and then temperature adjustment to precipitate the wall forming compound and to form the microcapsule wall. Because of the non-sticky nature of the wall forming compounds, microencapsulation of single droplets of oil is easily obtained, as opposed to the clusters and aggregates generally obtained by coacervation. The wall thickness of the microcapsules is not limited as in some prior art processes since the microcapsule wall is formed by precipitation of the wall forming compound and by the interaction of the wall forming compound wtih the cross-linking agent. The wall thickness can be controlled by the oil droplet size and relative amounts of wall forming compound and oil to be microencapsulated.
By the process of this invention, dispersions of approximately 50% microcapsules have been produced as compared to the more usual 25% obtained by prior art methods involving coacervation. Thus, the dispersion of microcapsulation can be used without dewatering in coating composition where a high solids content is desirable.
Other patents considered relevant although inferior to the product and process of this disclosure are: